The medullary folds are apparently still slightly open for the greater part of their length, though they are evidently fused together in the head region, except at the extreme end. Transverse sections, however, of [Figure 12], in which the medullary folds, from the dorsal aspect, seemed open (mg) as in [Figure 10], have shown that these folds are fused throughout their length.
The first cerebral vesicle (v′) is clearly indicated as an enlargement of the anterior end of the nervous system, and a slight enlargement (v″) posterior to the first probably represents the second cerebral vesicle.
There are now eight pairs of somites (s).
The head-fold (h) now shows in both dorsal and ventral views, appearing in the former, when viewed by transmitted light, as a lighter, circular area on either side of the body, just posterior to the hinder edge of the amnion.
The head-fold of the amnion (a) has extended about twice as far backward as it did in the preceding stage.
Owing to the opacity caused by the medullary folds being in contact along the middle line, the notochord is no longer visible in surface views.
The head at this stage begins to push down into the yolk in a strange way that will be described later.
STAGE VIII
Figures 11-11k ([Plates XVI.], [XVII.], [XVIII.])
This stage is about one fourth longer than the preceding. The medullary canal is enclosed throughout its entire length, though it appears in surface view ([Fig. 11]) to be open in the posterior half (mc) of the embryo. An enlargement of this apparently open region at the extreme posterior end (pg) is probably caused by the remains of the primitive groove or the neurenteric canal, and a slight opacity at the same point may be caused by the primitive streak. The anterior end of the neural tube is bent in a ventral direction (v), as in the preceding stage. The somites (s) now number fifteen pairs; they are somewhat irregular in size and shape.